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Circuit Function & Benefits

High-side current monitors are likely to encounter overvoltage conditions from transients or when the monitoring circuits are connected, disconnected, or powered down. This circuit, shown in Figure 1, uses the overvoltage protected ADA4096-2 op amp connected as a difference amplifier to monitor the high-side current. The ADA4096-2 has input overvoltage protection, without phase reversal or latch-up, for voltages of 32 V higher than and lower than the supply rails.

The circuit is powered by the ADP3336 adjustable low dropout 500 mA linear regulator, which can also be used to supply power to other parts of the system, if desired. Its input voltage can range from 5.2 V to 12 V when set for a 5 V output. To save power, the current sensing circuit can be powered down by removing power to the ADP3336; however, the power source, such as a solar panel, can still operate.

This applies voltage to the inputs of the unpowered ADA4096-2; however, no latch-up or damage occurs for input voltages up to 32 V. If slower throughput rates are required, the AD7920 can also be powered down between samples. The AD7920 draws a maximum of 5 μW when powered down and 15 mW when powered up. The ADA4096-2 requires only 120 μA under operational conditions. When operating at 5 V, this is only 0.6 mW. The ADP3336 draws only 1 μA in the shutdown mode.

Figure 2. ADA4096-2 Simplfied Schemactic

Circuit Description

The circuit is a classic high-side current sensing circuit
topology with a single sense resistor. The other four resistors
(dual 1 kΩ/20 kΩ divider) are in a thin film network (for ratio
matching) and are used to set the difference amplifier gain. This
will amplify the difference between the two voltages seen across
the sense resistor and reject the common-mode voltage:

VOUT = (VA – VB) (20 kΩ/1 kΩ)

Figure 2 shows a simplified schematic of the ADA4096-2. The
input stage comprises two differential pairs (Q1 to Q4 and Q5
to Q8) operating in parallel. When the input common-mode
voltage approaches VCC - 1.5 V, Q1 to Q4 shut down as I1
reaches its minimum voltage compliance. Conversely, when the
input common-mode voltage approaches VEE + 1.5 V, Q5 to Q8 shut down as I2 reaches its minimum voltage compliance. This
topology allows for maximum input dynamic range because the
amplifier can function with its inputs at 200 mV outside the rail
(at room temperature).

As with any rail-to-rail input amplifier, VOS mismatch between
the two input pairs determines the CMRR of the amplifier. If
the input common-mode voltage range is kept within 1.5 V of
each rail, transitions between the input pairs are avoided, thus
improving the CMRR by approximately 10 dB.

The ADA4096-2 inputs are protected from input voltage
excursions up to 32 V outside each rail. This feature is of
particular importance in applications with power supply
sequencing issues that could cause the signal source to be active
before the supplies to the amplifier are applied.

Figure 3 shows the input current limiting capability of the
ADA4096-2 provided by low RDSON internal series FETs (green curves) compared to using a 5 kΩ external series resistor with an unprotected op amp (red curves).

Figure 3. Input Current Limiting Capability

Figure 3 was generated with the ADA4096-2 in a unity-gain
buffer configuration with the supplies connected to GND (or
±15 V) and the positive input swept until it exceeds the supplies
by 32 V. In general, input current is limited to 1 mA during positive overvoltage conditions and 200 µA during negative
undervoltage conditions. For example, at an overvoltage of
20 V, the ADA4096-2 input current is limited to 1 mA,
providing a current limit equivalent to a series 20 kΩ resistor.
Figure 3 also shows that the current limiting circuitry is active
whether the amplifier is powered or not.

The AD7920 is a 12-bit, high speed, low power, successive
approximation ADC. The part operates from a single 2.35 V
to 5.25 V power supply and features throughput rates up to
250 kSPS. The part contains a low noise, wide bandwidth track-and-
hold amplifier that can handle input frequencies in excess
of 13 MHz.

The conversion process and data acquisition are controlled
using CS and the serial clock, SCLK, allowing the devices to
interface with microprocessors or DSPs. The input signal is
sampled on the falling edge of CS, and the conversion is
initiated at this point. There are no pipeline delays associated
with the part.

The AD7920 uses advanced design techniques to achieve very
low power dissipation at high throughput rates as described
below.

To enter power-down mode, the conversion process must be
interrupted by bringing CS high anywhere after the second
falling edge of SCLK, and before the tenth falling edge of SCLK.
Once CS is brought high in this window of SCLKs, the part
enters power down mode, the conversion that was initiated by
the falling edge of CS is terminated, and SDATA goes back into three-state. If CS is brought high before the second SCLK falling edge, the part remains in normal mode and does not
power down. This avoids accidental power down due to
glitches on the CS line.

To exit this mode of operation and power up the AD7920 again, a dummy conversion is performed. On the falling edge of CS, the device begins to power up, and continues to power up as long as CS is held low until after the falling edge of the tenth SCLK. The device is fully powered up once 16 SCLKs have
elapsed, and valid data results from the next conversion.

If CS is brought high before the tenth SCLK falling edge, the
AD7920 goes back into power-down mode again. This avoids
accidental power up due to glitches on the CS line or an
inadvertent burst of eight SCLK cycles while CS is low.
Although the device can begin to power up on the falling edge
of CS, it powers down again on the rising edge of CS, as long as
it occurs before the tenth SCLK falling edge.

Further details regarding the timing can be found in the
AD7920 data sheet.

Test Results

An important measure of the performance of the circuit is the
amount of noise in the final output voltage measurement.

Figure 4 shows a histogram of 10,000 measurement samples.
This data was taken with the CN-0241 Evaluation Board
connected to the EVAL-SDP-CB1Z System Demonstration
Platform (SDP-B) evaluation board. Details of the setup are
described in the Circuit Evaluation and Test section of this
circuit note.

The power supply was set to 3.0 V, and 10,000 samples of data
were acquired at the maximum rate of 250 kSPS without having
turned the output of the LDO off. Figure 4 shows the results of
this acquisition. The peak-to-peak noise is approximately
2 LSBs, corresponding to about 0.3 LSB rms.

Common Variations

The circuit is proven to work with good stability and accuracy.
This board is also compatible with the System Demonstration
Platform SDP-S Controller Board (EVAL-SDP-CS1Z).

A slight modification to the circuit shown in Figure 1 allows
monitoring the current for input supply voltages up to +30 V.
Rather than connect the +V pin of the ADA4096-2 to the +5 V
from the ADP3336, connect it directly to the input supply being
monitored. In this configuration, the ADA4096-2 is powered
directly from the input supply.

Circuit Evaluation And Test

This circuit uses the EVAL-CN0241-SDPZ circuit board and
the EVAL-SDP-CB1Z System Demonstration Platform SDP-B
Controller Board. The two boards have 120-pin mating
connectors, allowing for the quick setup and evaluation of the
circuit’s performance. The EVAL-CN0241-SDPZ board contains
the circuit to be evaluated, as described in this note, and the
SDP-B controller board is used with the CN0241 evaluation
software to capture the data from the EVAL-CN0241-SDPZ
circuit board.

Equipment Needed

PC with a USB port and Windows® XP or Windows Vista®
(32-bit), or Windows® 7 (32-bit)

EVAL-CN0241-SDPZ circuit evaluation board

EVAL-SDP-CB1Z SDP-B controller board

CN0241 SDP evaluation software

DC power supply capable of driving 6 V/1 A

DC power supply capable of driving 5 V/2.5 A

2Ω/12 W load resistor

Getting Started

Load the evaluation software by placing the CN0241 Evaluation
Software disc in the CD drive of the PC. Using "My Computer,"
locate the drive that contains the evaluation software.

Functional Block Diagram

See Figure 1 of this circuit note for the circuit block diagram
and the file “EVAL-CN0241-SDPZ-SCH-RevA.pdf ” for the
circuit schematics. This file is contained in the CN0241 Design
Support Package.

Setup

Connect the 120-pin connector on the EVAL-CN0241-SDPZ
circuit board to the connector marked “CON A” on the
EVAL-SDP-CB1Z controller (SDP-B) board. Nylon hardware
should be used to firmly secure the two boards, using the holes
provided at the ends of the 120-pin connectors. With power to
the supply off, connect a +6 V power supply to the pins marked
“+6 V” and “GND” on the board. If available, a +6 V "wall wart"
can be connected to the barrel connector on the board and used
in place of the +6 V power supply. Connect the USB cable
supplied with the SDP-B board to the USB port on the PC.
Note: Do not connect the USB cable to the mini USB connector
on the SDP-B board at this time.

Turn the 5 V/2.5 A dc supply on when data is ready to be
acquired. Adjust the voltage output accordingly to output the
amount of current you wish to measure.

Figure 6 shows a screenshot of the CN0241 SDP evaluation
software interface, and Figure 7 shows a screenshot of the
EVAL-CN0241-SDPZ evaluation board. Information regarding
the SDP-B board can be found in the SDP-B User Guide.

Test

Apply power to the +6 V supply (or “wall wart”) connected to
the EVAL-CN0241-SDPZ circuit board. Launch the evaluation
software and connect the USB cable from the PC to the USB
mini-connector on the SDP-B board.

Once USB communications are established, the SDP-B board
can now be used to send, receive, and capture serial data from
the EVAL-CN0241-SDPZ board.

Turn the 5 V/2.5 A dc supply on when data is ready to be
acquired. Adjust the voltage output accordingly to output the
amount of current you wish to measure.

Figure 6 shows a screenshot of the CN0241 SDP evaluation
software interface, and Figure 7 shows a screenshot of the
EVAL-CN0241-SDPZ evaluation board. Information regarding the SDP-B board can be found in the .

Pricing displayed is based on 1-piece. The USA list pricing shown is for budgetary use only, shown in United States dollars (FOB USA per unit), and is subject to change. International prices may vary due to local duties, taxes, fees and exchange rates.

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